This invention relates to improvements in organic semiconductor materials, and more particularly to improvements in thin film transistors (TFT) devices and thin film lasers with organic semiconductors.
Over the last decade, IC technologies have been proposed that use organic semiconductor thin film transistors (TFTs). The chief attractions of such circuits stem from the anticipated ease of processing and compatibility with flexible substrates. These advantages are expected to translate into a low-cost IC technology suitable for applications such as smart cards, electronic tags, and displays.
TFT devices are described in F. Garnier et al., Science, Vol. 265, pp. 1684-1686; H. Koezuka et al., Applied Physics Letters, Vol. 62 (15), pp. 1794-1796; H. Fuchigami et al., Applied Physics Letters, Vol. 63 (10), pp. 1372-1374; G. Horowitz et al., J. Applied Physics, Vol. 70(1), pp. 469-475; and G. Horowitz et al., Synthetic Metals, Vol. 42-43, pp. 1127-1130. The devices described in these references are based on polymers or oligomers as the active materials, in contrast with the amorphous silicon and polysilicon TFT structures that were developed earlier. The devices are typically field effect transistors (FETs). Organic active devices have significant advantages over earlier semiconductor TFTs in terms of simplicity of processing and resultant low cost. They are also compatible with polymer substrates used widely for interconnect substrates. Organic TFTs are potentially flexible, and polymer TFT ICs can be formed directly on flexible printed circuit boards. They also have compatible coefficients of thermal expansion so that solder bonds, conductive expoxy bonds, and other interconnections experience less strain than with semiconductor IC/polymer interconnect substrate combinations. While metal-insulator-semiconductor (MIS) FET devices are most likely to find widespread commercial applications, TFT devices that utilize both p-type and n-type organic active materials are also known. See e.g., U.S. Pat. No. 5,315,129. S. Miyauchi et al., Synthetic Metals, 41-43 (1991), pp. 1155-1158, disclose a junction FET that comprises a layer of p-type polythiophene on n-type silicon.
Recent advances in organic based TFT devices are described in U.S. Pat. No. 5,596,208, issued May 10, 1996, U.S. Pat. No. 5,625,199, issued Apr. 29, 1997, and U.S. Pat. No. 5,574,291, issued Nov. 12, 1996. With the development of both n-type and p-type active polymer materials, as described in these patents, complementary ICs can be readily implemented, as detailed particularly in U.S. Pat. No. 5,625,199.
With the basic organic TFT technology now well established, refinements in the materials used to make these structures can be expected. In particular, advances in semiconductor materials are continually being sought to improve the electrical performance of these devices. Higher mobility semiconductors would be especially useful.
To date, most p-channel organic semiconductor are based on thiophene derivatized oligomers and polymers. In addition, there are a few other systems based on aromatic macrocyclics, such as metallopthalocyanines, and fused aromatic ring structures such as pentacene and tetracene. Very few other types of conjugated systems have been explored. In addition, most of the reported p-channel semiconductors have relatively low bandgap and high HOMO level. Therefore they tend to be easily photo-oxidized which causes degradation of the resulting devices.
Recent work with fluorene conjugated materials has shown useful semiconductor characteristics. See Sirringhaus et al., Applied Physics Letters, Vol. 77, No. 3, July 2000. However, the fluorene material reported by these workers is produced by a liquid crystal (LC) technique and requires alignment of LC domains using a special transistor structure developed for this LC material. Such an approach complicates transistor fabrication and adds cost.
Fluorene based organic semiconductor materials have been developed that show improved transistor performance and do not require either special preparation or special transistor structures. Generally speaking, these materials are fluorene-containing oligomers. Due to high steric hindrance between fluorene and other co-oligomer units, the resulting oligomers have relatively high bandgaps and low HOMO levels. These semiconductors have shown high on/off ratios with good aging characteristics. In addition, the bandgap and emission wavelengths of these materials can be easily tuned by changing the co-oligomer unit structure. The ability for color tuning the emission wavelength of the organic semiconductor, while maintaining high carrier mobility gives rise to multicolor electrically injected organic semiconductor lasers.
These materials can be prepared as polycrystalline thin films that do not require crystallographic alignment to give good electrical performance.